Materials and methods are described to produce xylitol from a mixture of hemicellulosic sugars by several routes. Examples include either as a direct co-product of a biorefinery or ethanol facility, or as a stand-alone product produced from an agricultural or forestry biomass feedstock including using, e.g. ethanol waste streams.
Xylitol has several favorable properties as a sugar substitute, such as low caloric content, anticariogenicity, good gastrointestinal tolerance, and near insulin-independent metabolism in humans. The traditional production of xylitol involves direct chemical hydrogenation of hemicellulosic hydrolysates over a Raney-Nickel catalyst followed by extensive purification from non-specific reduction products. In the chemical process, D-xylose is converted to xylitol by catalytic reduction. This method utilizes specialized and expensive equipment for the high pressure and temperature requirements as well as the use of a Raney-Nickel catalyst that can introduce trace nickel into the final product, which is undesirable. Additionally, the overall yield is only 50-60%. The final product must also be purified. This multi-step process is expensive and inefficient.
Hydrolysate from birch trees has historically been the only economic source of xylose used to make xylitol by chemical hydrogenation. Birch tree hydrolysate is a byproduct of the paper and pulping industry and it has only minor amounts of arabinose and other sugars. However availability severely limits this source of xylitol. Hydrolysis of other xylan-rich materials, such as trees, straws, corncobs, oat hulls under alkaline conditions also yields hemicellulose hydrolysate, however these hydrolysates contain too many sugars other than xylose, especially L-arabinose. These competing sugars create a number of by-products during the hydrogenation process that are difficult and costly to remove.
Biocatalytic routes to xylitol production using fungal or yeast xylose reductase (XR) have also been explored. Unfortunately, the nonspecific nature of direct hydrogenation is only partially addressed in the biocatalytic route. The natural promiscuity of XRs toward other sugars, particularly L-arabinose, another major component of hemicelluloses, necessitates the prior purification of D-xylose to minimize formation of L-arabinitol. Because D-xylose and L-arabinose are epimers, their separation is nontrivial, and is one of the leading obstacles to the more economical production of xylitol.
Because there is a significant amount of arabinose in the hydrolysates, a significant amount of arabinitol (arabitol) is produced because the xylose reductase enzyme that converts xylose to xylitol also converts arabinose to arabinitol. A significant challenge was to develop either a process that produces negligible amounts of arabinitol or alternatively converts the arabinose into additional xylitol.
While some basic research has been performed by others in the field, development of an effective bioprocess for the production of xylitol has been elusive. Many of these systems suffered from problems such as poor microbial strain performance, low volumetric productivity, and too broad of a substrate range. Moreover, kinetics and overall performance of the enzymes reported to date have not been engineered (via methods such as directed evolution) to maximize efficiency. More efficient enzyme activity would result in improved throughput and shorter reaction times, both of which are crucial to a commercially viable process.
Most of the research performed has also been carried out using a highly purified and concentrated D-xylose substrate. This substrate has no significant amounts of other pentoses such as arabinose or other hexoses such as D-glucose. While some reasonable yields with such a substrate have been reported, developing a bioprocess with pure D-xylose is impractical due to the cost of this substrate and the fact that it can be hydrogenated at similar costs and better space-time yields.
None of the approaches described in this section have been commercially effective for a number of reasons. First, xylose uptake is often naturally inhibited by the presence of glucose that is used as a preferred carbon source for many organisms. Second, none of the enzymes involved have been optimized to the point of being cost effective. Finally, xylose in its pure form is expensive and any requirement for a bioprocess to use pure xylose results in direct competition with inexpensive chemical hydrogenation. Additionally, all of the systems developed would produce arabinitol as a significant contaminating byproduct since the xylitol dehydrogenase used has similar activity with both xylose and arabinose.
Xylitol could potentially be a byproduct of ethanol production. When products such as ethanol or other chemicals are produced from corn by current processes, only starch is generally utilized. Thus, during ethanol production, significant by-products rich in pentose and other sugars are made. For example, when ethanol is produced from a dry-mill operation (about 55% of the facilities today) distiller's dry grains (DDG) and other byproducts are produced. In the wet-mill operation (the remaining 45% of current facilities) corn fiber rich in hemicellulose is produced. These products are usually sold as inexpensive animal feed or otherwise disposed of, but both the corn fiber and distiller's dry grains could potentially be converted to other value-added products, such as xylitol which could help improve the economics of ethanol production.